
# enfoce airline requirement
# for every node i:
#	the sum of itinerary[edge] over all outgoing edges/flights operated by any airline
#	in the preferred set must be atleast as large as the sum over all outgoing
#	edges/flights.
# Essentially: the outgoing edge, if any, must be from a preferred airline or none at all

subject to c_enforce_airline_pref {i in nodes}:
sum {aln in preferred_airlines, fl_num in node_outgoing_airline_flights[i,aln], (i,dest_node[fl_num],fl_num,l) in ARCS} itinerary[i,dest_node[fl_num],fl_num,l]
>= sum {fl_num in node_outgoing_flights[i], (i,dest_node[fl_num],fl_num,n) in ARCS} itinerary[i,dest_node[fl_num],fl_num,n];
